The objective of this proposal is to examine the mechanisms involved in thyroid hormone activation of gene expression with particular emphasis on the interaction of the thyroid hormone receptor (TR) with the potential transcriptional coactivator TRIP1 (thyroid hormone receptor interacting protein 1). TRIP1 shares very strong sequence identity with the yeast trancriptional coactivator, SUG1. Therefore, the hypothesis is that TRIP1 functions as a TR-specific transcriptional coactivator. The domains that are responsible for the TR-TRIP1 interaction will be identified using the GST-fusion pulldown assay and site-directed mutagenesis. The influence of hormone on the TR-TRIP1 interaction will be examined. Lastly, the functional significance of the TR-TRIP1 interaction with respect to gene regulation will also be investigated. Mammalian cell transfections and an in vitro transcription assay will be employed. Examination of the role of TRIP1 during TR-mediated gene regulation is very important because thyroid hormone is one of the most prescribed, therapeutic agents.